1. Field of the Invention
This invention relates to electrodialysis apparatus including alternated layers of metal electrodes, flow spacers, and permselective membranes, all pressed tightly together in a stack. In such apparatus different liquids under pressure flow through separate chambers. As a consequence, leak-free operation is an essential requirement.
2. Description of the Prior Art
Electrodialysis apparatus has been used for the concentration or the demineralization of electrolytes from diverse solutions containing electrolytes such as saline water, electroplating waste, radioactive waste, organic acid solutions and the like. In such electrodialysis apparatus, it is quite important to prevent leakage of the different liquids from one cell compartment to another since such leakage may cause contamination of the dialyzed liquid and render the electrodialysis operation inoperable or diminish its efficiency. It is important, also, to prevent external leakage, as from the areas between the cell compartments, particularly where the liquids may have acid, alkali or other toxic properties contaminating to the environment.
Various proposals have been made in the prior art for preventing leakage of the liquids between and from cell compartments in electrodialysis apparatus. Thus, the use of multi-ply membrane spacers is disclosed in U.S. Pat. No. 3,878,086 issued to David B. Haswell et al. on Apr. 15, 1975 and in U.S. Pat. No. 4,319,978 issued to William G. Millman on Mar. 16, 1982. In U.S. Pat. No. 3,878,086 the outer plies are selected to be more rigid than those in the interior and are held together by a water-resistant pressure-sensitive adhesive. The resultant spacer is adhesively bonded to membranes on each of the sides thereof. The plies in U.S. Pat. No. 4,319,978 are held together by a water-resistant pressure sensitive adhesive with the outer plies being made of compressible material and those in the interior of more rigid material. The spacer so formed is assembled in a stack without being adhesively bonded to the membranes thus facilitating disassembly of the stack for servicing without tearing of the membranes that tends to result when the membranes are adhesively bonded to the spacer as in U.S. Pat. No. 3,878,978.
In U.S. Pat. No. 3,933,617 issued to Tosinori Yamamoto et al., low leakage of liquid between the cells of a filter-press type electrodialysis apparatus is attributed to the feeding and discharge means of the cell being formed separately from the body of the cell frame and made of a rigid material having a thickness preferably less than that of the cell frame. The cell frame is formed of elastic material whereby, upon assembly, the cell frames are compressed while the feeding and discharge means are substantially noncompressed.
U.S. Pat. No. 4,217,200 issued to Ora Kedem et al. on Aug. 12, 1980 discloses a membrane unit for use in electrodialysis apparatus comprising a heat-sealable frame framing and forming an integral entity with an electrodialysis membrane.
U.S. Pat. No. 4,303,493 issued to Klemens Kneifel et al. on Dec. 1, 1981, discloses sealing layers or coatings of foamed epoxy resin applied by a screen printing method adjacent or adjoining sealing surfaces of the sealing frame such as the chamber bores and supply and discharge channels.
In U.S. Pat. No. 4,600,493 granted on July 15, 1986, to Emmanuel Korngold for Electrodialysis Apparatus for the Chemical Maintenance of Electroless Copper Plating Baths and assigned to the assignee of the present invention, the disclosure of which patent, by reference, is incorporated herein, there is disclosed an electrodialysis apparatus comprising alternated layers of metal electrodes, Teflon flow spacers and thin Teflon gaskets, all pressed tightly together. (The generic name for Teflon, which is a trademark of E. I. DuPont deNemours and Company, is tetrafluoroethylene.) High pressure liquds of three types flow through the separate compartments of the apparatus, the Teflon gaskets being provided to preclude leakage.
Such prior art arrangements for achieving leak-free operation are characterized by a number of disadvantages. They are overly complicated requiring additional components such as separate gaskets or other sealing layers or complicated structures including, in some cases, water-resistant pressure sensitive adhesives that add undesirably to the cost of manufacture and assembly and disassembly when necessary for servicing, and moreover, tend to increase the likelihood of operation failure.
Thus, there exists a need and a demand in the art for simplification and improvement in electrodialysis apparatus to the end of achieving leak-free operation which is necessary for the successful commercial operation of the apparatus. The present invention was devised to fill the technological gap that exists in this respect.